Heretofore, optical sensing systems have been used in various measuring devices for accurately measuring the relative movement of two objects. These systems typically have employed photoelectric detectors and have utilized two or more optical gratings to control a light-dark measurement pattern sensed by the photoelectric detectors. Usually, the light-dark pattern is in the form of an optical interference fringe signal, commonly referred to as a Moire fringe. This fringe signal is a very gradual light variation from a condition of full illumination to a condition of full darkness. An example of such a system can be found in U.S. Pat. No. 3,796,498 wherein a movable scale grid travels parallel to and in close proximity with a stationary index grid and the resultant moire fringe pattern is focused on a photoelectric detector. However, it has been recognized that dirt, smudges, scale wear or similar defects in or on the scale grid, can cause substantial error and that such an error is magnified where only a small gap is maintained between the two grid members. The more recently issued U.S. Pat. No. 4,079,252 recognized some of the problems associated with the narrow gap arrangement used thereto fore and described a measuring apparatus utilizing a "fringe pattern" that provided a considerable gap between the grid and a lens for focusing the resulting fringe pattern onto the detector. Of course, as will be appreciated, the focused pattern is still a spacially periodic distribution of radiation with a substantial variation of light intensity. This spatial variation of light intensity is typical of sinusoidal character and the detector must focus on and respond to only a small protion of the spatial light intensity cycle to detect a substantial contrast between the highest intensity areas of light and darkness.
Another device using an optical measuring system employing a fringe pattern is disclosed in U.S. Pat. No. 3,578,979 wherein the index grid is spaced substantially from a scale grid, with the former grid being formed directly on one surface of the photoelectric element. In this device direct specular reflection, rather than diffuse or indirect reflection for illumination of the scale grid, is required in order to provide proper definition of the scale image projected onto the index grid. Of course, as can be appreciated, in a system of that type the photodetector must be quite large and accurate placement of the index grid on the detector is extremely critical. Even a very slight manufacturing variation will adversely affect the operation of the device. This is particularly true if that system were used with a flexible blade whose lateral position shifts and may allow ambient light into the system.